The invention relates to an optical waveguide sensor, for example, for use as a chemical or biological sensor.
The evanescent wave portion of an electromagnetic field propagating through an optical waveguide characteristically penetrates up to several hundred nanometers into the medium surrounding the optical waveguide. This evanescent wave can excite fluorescent molecules, e.g., fluorophores, to fluoresce when these molecules are near the optical waveguide surface, within the depth of penetration. The application of this phenomenon to an immunoassay sensor, wherein the biological recognition (binding) of antigen to antibodies attached to the waveguide surface with the concomitant displacement of fluorescent-labelled antigen is measured as a change in fluorescence, was first disclosed in "A New Immunoassay Based on Fluorescence Excitation by Internal Reflection Spectroscopy" by Kronick and Little, Journal of Immunological Methods 1975, Vol. 8, page 235.
The use of optical fibres as a special class of waveguides for immunoassay sensors is also known. For example, U.S. Pat. No. 4,447,546 discloses the use of optical fibers as waveguides which capture and conduct fluorescence radiation emitted by molecules near their surfaces.
U.S. Pat. No. 5,061,857 describes an optical sensor in which the end of a multimode optical fiber is tapered. The evanescent field excites fluorescence which is coupled back into the optical fiber and returns to a detector. The problem with this approach is that the detector is relatively inefficient in capturing the fluorescence.